System and method for a fast carry/sum select adder

Abstract

An adder system includes at least one adder block subsystem. Each adder block subsystem includes a pair of input signal lines, an adder circuit block having a conditional sum-select and a conditional carry-select, a sum-high line, a sum-low line, a carry-high line, carry-low line, a sum selection switch, a carry selection switch, a carry forward line, and an output signal line. The input lines are individual bit lines that are paired together from the least significant bit to the most significant bit. Within the adder circuit block, pairs of the input bit lines are coupled to the conditional sum-select and the conditional carry-select. The conditional sum-select is coupled to the sum-high and sum-low lines and the conditional carry-select is coupled to the carry-high and carry-low line. The sum selection switch selectively couples the output signal line to the sum-high or the sum-low line. The carry selection switch selectively couples the carry-high or carry-low line to the carry forward line that is coupled to the next adder subsystem which is structured similarly. The carry signal of the adder system is determined through the conditional carry selects of the adder circuit blocks. Each conditional carry-select includes logic OR and logic AND subcircuits to which each of the pairs of input bit lines is respectively coupled. The outputs of the logic OR and logic AND subcircuits are coupled to one or more line multiplexes to selectively produce a logic high or logic low signal that serves as the individual carry bit signals for the carry signal. A method for an adder system is also disclosed.